Balanced mixers generally have many advantages in wireless communication systems and they are applied to diverse fields. In the first place, balanced mixers have an excellent isolation property between ports of a radio frequency (RF) unit and a local oscillator (LO). Secondly, balanced mixers have a wideband characteristic. Thirdly, they have a wide dynamic range These advantages have made the balanced mixers applicable to diverse RF systems. Herein, the isolation property is one of important factors for assessing the performance of a mixer. Isolation signifies the extent of isolating two certain objects. Generally, isolation mentioned in a wireless system is a parameter indicating the extent of prohibiting interference caused in an adjacent port to a port of an input signal or an output signal from affecting another port.
Isolation will be described in detail hereafter by taking an example of a wireless communication system employing a widely used superheterodyne scheme. In a super-heterodyne wireless communication system, RF signals are converted into intermediate frequency (IF) signals by using signals generated in a local oscillator. Thus, the super-heterodyne wireless communication system uses a mixer having three ports: one for IF signals, one for signals from local oscillator, and one for RF signals. Since the mixer is a device for frequency conversion, the isolation among the three ports becomes a particularly important parameter.
Balanced mixers include single balanced mixers and double balanced mixers. The single balanced mixers have a structure of isolating two frequency input units that are considered to have an isolation problem by using a coupler or a transformer. The two frequency input units are generally a port for RF signals and a port for signals from a local oscillator. In other words, when the two inputs are distinguished by using a coupler or a transformer, signals transmitted to a counterpart becomes minimized although signals are simultaneously inputted from both units. Herein, the coupler may be a 90 hybrid coupler or a coupler giving one part a 180° phase difference.
Double balanced mixers are devised to enhance the isolation more than the single balanced mixers. In short, the double balanced mixers have a structure connecting all input output units by a balun to completely isolate all ports one from anther. A balun is an apparatus for dividing one input signal into two signals having the same signal amplitude and a phase difference of 180° from each other. A structure of a double balanced mixer will be described hereafter.
Generally, a balun having excellent characteristic is designed in a distribution type, which has a shape of microstrip. A representative balun having the microstrip-shaped distribution type is a Marchand balun. A Marchand balun includes two coupled lines each having a length of ¼ wavelength (λ) and using a medium part of a connection line with its both ends grounded as a load so as to realize a wideband balun. The frequency band of a Marchand balun is known to range to a frequency twice the initial frequency, which is 1 octave.
A disadvantage of the Marchand balun is that the positions of ports are not flexible. This is because since the medium part of the connection line with two output ports is used, the two output ports are positioned close to each other. Thus, it may be advantageous to use a Marchand balun in a double balanced mixer if the double balanced mixer has a special structure where output ports are gathered in a star shape. However, if the double balanced mixer has a structure where the output ports are apart from each other, it is difficult to apply the Marchand balun to the double balanced mixer.
To overcome the problem, Y. C. Leong suggested a balun of a modified form in a paper entitled “A Derivation of a Class of 3-Port Baluns from Symmetrical 4-Port Networks,” IEEE MTT-s Digest, 2002, pp. 1165-1168. Y. C. Leong proposed a balun positioned in the medium part of a connection line with ¼ wavelength and having a structure where both ends of an input port are grounded and two outputs are disposed at both ends of the connection line. With the balun structure proposed by Leong, sufficient space can be acquired between the two output ports. Thus, positional flexibility is acquired even in a structure where output ports are positioned apart from each other.
Hereafter, the double balanced mixer described above will be described in detail with reference to the accompanying drawings.
FIG. 1 illustrates a typical double balanced diode mixer employing a two-way balun with an LO balun and an RF balun.
The typical double balanced diode mixer includes three ports. One is an input port 102 of an RF balun 105, and another is an input port 103 of the LO balun 106. The other is an IF output port 104. The output of the LO balun 106 is outputted through two ports. Also, a diode unit 101 includes four diodes D1, D2, D3 and D4 in a structure where the anodes and cathodes are connected to each other in a ring shape. One output port of the LO balun 106 is connected to an anode of the diodes having a ring shape, and the other output port is connected to an anode of an opposite diode of the diodes having a ring shape. Also, to have a look at how the LO balun 106 is connected in FIG. 1, one output port of LO balun 106 is connected to an anode of a first diode D1 and another input is connected to an anode of a third diode D3.
The RF balun 105 also includes two output ports. One output port is connected to an anode of another diode which is not connected to the LO balun 106 among the diodes of the ring shape, and the other output port is connected to an anode of an opposite diode. Referring to FIG. 1, one output port of the RF balun 105 is connected to an anode of a second diode D2 whereas the other output port is connected to an anode of a fourth diode D4. The anode of the fourth diode D4 is connected to the anode of the second diode D2 by two inductors L1 and L2, and an IF output port is connected between the two inductors L1 and L2.
The double balanced mixer having the above-described structure includes the RF balun 105, the LO balun 106, and a branch-shaped diode ring. The RF balun 105 and the LO balun 106 are formed in the RF port 102 and the LO port 103 to output signals having a phase difference of 180° from each other. Therefore, signals with a phase difference of 180° are inputted to each diode pair. In short, the two outputs of the double balanced mixer are signals having the same amplitude and a phase difference of 180° to each other.
The double balanced mixer shown in FIG. 1 has an overlapping section 107 where lines are overlapped. According to the characteristics of a balun and a double balanced mixer, a typical double balanced mixer has a part where one of the output lines of the LO balun 106 is overlapped with one of the output lines of the RF balun 105 at one point. When the two signal lines are overlapped at one point, a short is avoided by using a two-layer substrate or a jump line. When a board-type circuit is designed and signal lines are overlapped, an additional process of fabricating a two-layer substrate or a jump line is needed to avoid a short, and this increases the production costs. Also, the overlapped part where the two lines are superposed may cause an RF parasitic effect in the double balanced mixer. When the RF parasitic effect occurs, the characteristics of the balun are deteriorated to thereby degrade the overall characteristics of the double balanced mixer. FIG. 1 also shows another overlapping section in the signal line connected to the IF output port 104. However, as the frequency of IF signals are generally very low compared to RF or LO signals, the overlapping in the signal line connected to the IF output port 104 does not affect the RF characteristics greatly.
FIG. 2 illustrates a structure of a two-way balun disclosed in the reference literature. The two-way balun shown in FIG. 2 has a structure obtained by modifying part of a general wideband Marchand balun.
A third plane (P3) connected to an input port 201 has a length of ¼λ and the other end of the third plane (P3) is grounded. In FIG. 2, although the ground is given only one reference numeral, the grounds are the same. In case of a planar substrate, the ground is realized in the form of a via hole. A first plane (P1) having the same size as the third plane (P3) have its both ends grounded, and the first plane (P1) and the third plane (P3) are disposed in a row. A second plane (P2) disposed in parallel to the first plane (P1) and a fourth plane (P4) disposed in parallel to the third plane (P3) are connected in the shortest way. The other end of the second plane (P2) is a second output port (Out2), and the other end of the fourth plane (P4) is a first output port (Out1).
Signals outputted from the first output port (Out1) and the second output port (Out2) have the same amplitude and the opposite phase. A wideband balun is generally realized using coupled lines and an odd mode impedance and an even mode impedance become significant values deciding the characteristics of the balun in the designing of the balun. In the balun having the structure of FIG. 2, a conventional technology decides the odd mode impedance and the even mode impedance as expressed in Equation 1.
                                          Z            oo                    =                                                    -                                                      Z                    o                    2                                                              +                                                Z                  o                                ⁢                                                                            2                      ⁢                                              Z                        o                        2                                                              +                                                                  Z                        s                                            ⁢                                              Z                        L                                                                                                                                                                  Z                  s                                ⁢                                  Z                  L                                                                    ,                              Z            oe                    =                                    Z              o              2                                      Z              oo                                                          Eq        .                                  ⁢        1            
where ZS denotes impedance of a signal source; ZL denotes impedance of a load; Zo denotes a characteristic impedance of a coupled line; Zoo denotes an odd mode impedance; and Zoe denotes an even mode impedance.
The balun shown in FIG. 2 is eventually a two-way balun, and as described above, a two-way balun should have a circuit structure shown in FIG. 1. Therefore, the overlapping section where an output line of the LO balun and an output line of the RF balun are overlapped is induced. When the line overlapping section is caused, the problem associated with FIG. 1 cannot be resolved.